Many types of conventional electrode leads implantable in living bodies (to be referred to as implantable electrode leads hereinafter) which are used together with implantable heart pacemakers or implantable defibrillators, are known.
An implantable electrode lead is generally comprised of an electrode for supplying an electrical stimulation pulse to the heart or sensing an electrically evoked response of the heart, an electrical connector connected to a heart pacemaker (or defibrillator), and a lead body made up of an electrical conductor wire for connecting the electrode and electrical connector to each other and transmitting an electrical signal between the electrode and heart pacemaker (or defibrillator) and an electrical insulation covering the electrical conductor wire.
In the implantable electrode lead, the electrode and part of the lead body are inserted in the heart and vein, while the remaining lead body and the electrical connector are placed outside the vein and connected to the connection housing of the heart pacemaker or implantable defibrillator.
As an example of a lead body for an implantable electrode lead, a bipolar type lead body in which a plurality of insulation-coated conductor are connected parallel to each other is used, as shown in Japanese Patent Laid-Open No. 11-333000. The conductor wires are wound parallel to each other insulated from each other so as to form a coil, and the conductor wires are covered with a sheath, thus forming a lead body.
According to another general lead body, one with a coaxial structure is available, which is comprised of two types of conductor coils with different average diameters, an insulating sheath located between the conductor coils, and a sheath located at the outermost surface of the lead body.
A bipolar type implantable electrode lead body comprised of a plurality of conductor wires with respect to one electrode is used because, even if one of the conductor wires is disconnected, as long as the remaining conductor wires are connected normally, an electrical signal from the heart pacemaker can be continuously transmitted to the living tissue.
As the implantable electrode lead, one with a small energy loss in the conductor wires is sought. For this reason, two conductor wires with the same low resistance are used.
In a conventional implantable electrode lead using two low-resistance conductor wires, even if one conductor wire is disconnected, this produces a small change in electrical resistance, as will be described later. Therefore, it is rather difficult to externally detect disconnection from a change in electrical resistance after the implantable electrode lead is implanted in the living body.
This will be described in detail with reference to FIGS. 14 and 15.
FIG. 14 shows a case in which a bipolar type implantable electrode lead, comprised of two low-resistance conductor wires connected parallel with each other, is connected to a living tissue. The lead body of an implantable electrode lead 60 is comprised of two portions, i.e., a tip electrode-side lead resistor 61 and ring electrode-side lead resistor 62. The implantable electrode lead 60 and the living tissue 32 are connected to each other through a tip electrode 4 and ring electrode 3, and the living tissue 32 and the implantable electrode lead 60 substantially electrically form a series circuit as shown in FIG. 14. The implantable electrode lead 60 is connected to an implantable heart pacemaker through a connector pin 1 and connector ring 2.
Each of the tip electrode-side lead resistor 61 and ring electrode-side lead resistor 62 is comprised of two conductor wires with the same resistance (R1). To obtain the resistance R1 used for the implantable heart pacemaker, for example, a low-resistance conductor wire with approximately 16 Ω is used. A bioelectrical resistance 32 (R4) of the living tissue is approximately 1,000 Ω.
FIGS. 15A and 15B show a case wherein changes in resistance that occur when, of the four conductor wires, one conductor-wire of the tip electrode-side lead resistor 61 is disconnected with the condition described above are obtained. FIG. 15A shows changes in electrical resistance upon disconnection. The resistance of the tip electrode-side lead resistor 61 increases from 7.88 Ω to 15.76 Ω upon disconnection.
FIG. 15B shows a change in total electrical resistance obtained on the basis of FIG. 15A. Although the total electrical resistance after disconnection (1,023.63 Ω) is slightly larger than that (1,015.76 Ω) obtained before disconnection, the increase is small, approximately 1% (the ratio of total electrical resistance before disconnection to that after disconnection: 1.008). Also, since the bioelectical resistance sometimes fluctuates by approximately several 10 Ω, it is difficult to determine disconnection from the 1% increase in resistance.
As another lead body for the bipolar type implantable electrode lead, one with a coaxial structure is also available, which is comprised of two types of conductor coils with different average diameters, an insulating sheath located between the conductor coils, and a sheath located at the outermost surface of the lead body.
When the conductor wires of the same pole are not insulated from each other, as in the conventional coaxial structure, the adjacent conductor wires are largely influenced by the contact resistance. As the contact resistance varies depending on the movement or deformation of the lead in the living body, it is sometimes difficult to externally detect the contact resistance from a change in resistance.
When, however, an implantable electrode lead used in a heart pacemaker or implantable defibrillator is completely disconnected, the treatment necessary for the patient cannot be performed, sometimes leading to the worst result such as death of the patient. For this reason, it is desired that a symptom of disconnection of the implantable electrode lead be found at an early stage before complete disconnection.